An electrochemical sensor is known, for example, as a sensor for measuring an objective substance (specified component) contained in a sample. The electrochemical sensor is the sensor which is capable of detecting a minute amount of current by utilizing an electrochemical reaction. In the case of a glucose sensor of the subcutaneous retention type (to be retained subcutaneously beneath the skin), when glucose molecules, which exist in a specimen or sample, arrive at the enzyme developed on an electrode, glucose is oxidized in accordance with the enzymatic reaction. It is possible to estimate the concentration of glucose in a body fluid on the basis of a response current signal obtained by electrochemically oxidizing hydrogen peroxide (H2O2) which is produced during this process. The sample is exemplified, for example, by an intercellular fluid which exists extracellularly in the subcutaneous tissue. The method, in which the response current obtained by allowing glucose contained in a sample to react together with a reagent enzyme (for example, glucose oxidase (GOD), glucose dehydrogenase (GDH) or the like) provided on a measuring sensor as described above, is referred to as “enzyme electrode method”. Further, a sensor is also known, which adopts such an optical detecting method (referred to as “colorimetric method” in some cases) that the intensity is measured at a color developing wavelength while previously applying a color developing reagent enzyme (for example, hexokinase (HK) or the like) which specifically causes a reaction of an objective substance contained in a sample.
In this specification, a part or region, which is provided on a substrate of a measuring sensor and which is provided to detect an objective substance contained in a sample, is referred to as “detecting unit”. For example, the reaction reagent is retained in the detecting unit. The body fluid contains, for example, microorganisms (for example, bacteria and fungi), protein, fibrin, and lipid in addition to glucose as the objective substance. For example, when the microorganism exists around the detecting unit (for example, an electrode of a glucose sensor), it is feared that the measurement accuracy may be deteriorated by destroying the enzyme developed on the electrode and/or consuming, for example, glucose, oxygen or the like. In relation thereto, such a technique has been suggested that an antifungal agent, an antibiotic or the like is previously mixed with the enzyme (see, for example, Patent Document 1).
On the other hand, if the subcutaneous retention period is prolonged, then protein, fibrin and the like gradually adhere to the electrode, and the formation of foreign body capsule (FBC) is induced. Finally, matured FBC such as vascular fibrous tissue is formed (see, for example, Non-Patent Document 1). If such a situation arises, for example, glucose hardly arrives at the electrode. It is feared that any influence may be exerted on the measurement of the glucose concentration. In relation thereto, a biosensor has been also suggested, comprising a first film which is a porous film for covering an electrode and which has an electron donor region formed on a surface thereof for allowing protein and fibrin to easily adhere, and a second film which is combined with the electron donor region of the first film and which has a bonding hydrogen atom donor composed of phenyl ring (see, for example, Patent Document 2).